1. Field of the Invention
The invention relates to a data carrier with a data processing device as well as to an electronic component with a data processing device, for example for such a data carrier.
2. Description of the Related Art
Recently doubts have been raised as regards the security of data carriers, it being stipulated that security-relevant data could be determined by monitoring the power consumption of such data carriers. It is true that during all logic operations, so also during sensible operations or sub-operations (for example cryptographic calculations) current power is consumed by switching operations in the logic circuitry in dependence on the result or logic level. Measurement of the power consumed by the circuit, therefore, could be used for an attack so as to find out secret data (key) by means of mathematical methods (correlations, power analysis).
It is an object of the invention to prevent such attempts from being successful.
This object is achieved in that a load circuit is connected to the power supply of the data carrier and is intended to influence the power consumption of the data carrier at least during security-relevant operations of the data processing device.
The power consumption that can be measured externally thus no longer corresponds to the power consumption of the data processing device alone, but also contains a further component which preferably is not directly related to the internal operations of the data processing device.
A particularly simple embodiment is obtained when the load circuit is constructed as a variable ballast resistor which in the simplest case may consist of a transistor or a network of series and parallel-connected transistors, connected to the same power supply lead in parallel with the data processing device. Different load states can be adjusted by appropriate control of the load resistor or load resistors.
A more complex embodiment is provided with a circuit arrangement which is constructed so as to be complementary to at least parts of the data processing device and can be controlled in parallel with the data processing device. Changes in the switching state, initiated during security-relevant operations, are thus carried out in a complementary fashion at the same time. Even if the power consumption should be different for different logic levels, in the ideal case the power consumption is constant because of the complementary switching states. However, because it cannot be detected from the outside what power consumption relates to the logic states actually involved in the security-relevant operations and what power consumption is involved in the complementary switching states which occur in parallel merely for the purpose of masking, it is not even necessary to pursue a constant power consumption. Therefore, it is not even necessary to construct all switching circuit components required for the security-relevant operations in a complementary fashion, but it suffices to make only a part of the circuit components complementary.
Preferably, the load circuit and the data processing device are integrated in a common circuit because the separation of the load circuit from the data processing data for the purpose of attempted discovery requires far more technical means than when these circuit components are arranged on the data carrier in a physically separated manner. The analysis of circuit elements actually involved in security-relevant operations is rendered more complicated notably when the circuit elements required are physically mixed with complementary circuit elements in one chip.